1. Field of the Invention
The present invention relates to method and apparatus for engaging an engine clutch for a hybrid transmission and more particularly relates to the method and apparatus for engaging the engine clutch to start an engine upon request of the engine start during an electric vehicle run by means of only a motor/generator.
2. Description of the Related Art
A hybrid transmission is constituted by a differential device which mutually links among an engine, an output axle, and at least one motor/generator. Thus, an automotive vehicle can be made in an electric vehicle run mode only by means of a power from the motor/generator and can be run in a hybrid vehicle run by means of an engine power and the power from the motor/generator. Any way, a merit of using the hybrid transmission is damaged since the engine becomes a load during the electric vehicle run and a wasteful consumption of an energy is large if one of revolutional elements constituting the differential device which is associated with the engine is always coupled to the engine. Therefore, as described in a Japanese Patent Application First Publication No. 2003-034154 published on Feb. 4, 2003 (which corresponds to a U.S. Patent Application Publication No. U.S. 2003/0073534A1), such a technique that an engine clutch is interposed between one of the revolutional elements of the differential device which is associated with the engine and the engine and, during the electric (vehicle) run, this engine clutch is released so as to be separated between the engine and the hybrid transmission has been proposed. On the other hand, in the hybrid transmission, the electric vehicle run is used from a viewpoint of a smooth start and an easiness in control during the start of the vehicle. In a case where a large driving force is demanded and it is necessary to transfer the vehicular run into a hybrid (vehicle) run, an engagement of the engine clutch causes the engine to be started so that a transfer to the hybrid vehicle run is requested. When the engine is started during the transfer from the electric vehicle run to the hybrid vehicle run, the engine clutch is engaged to couple between the engine and the hybrid transmission. Thus, with the engine cranked and a fuel supplied to the engine, the engine is started.